The present invention relates to plasma-hydraulic excavation systems. In particular, the present invention relates to a plasma-hydraulic excavation system suitable for use in connection with mining operations, quarying and civil applications.
Conventional continuous mining techniques utilize mechanical fracturing and crushing as the primary mechanism for pulverizing rock. However, in hard rock applications the cutting edges of tools used in connection with mechanical fracturing and crushing require frequent replacement, and the overall efficiency of such methods is poor. In addition, significant pressure must be exerted against the face of the rock in order to achieve the desired fracturing, or cutting, using mechanical techniques.
In order to improve the speed and efficiency with which rock can be continuously excavated, mechanical techniques have been used in combination with explosives. According to such techniques, holes may be formed in the rock face using mechanical drills. Explosives may then be placed within the holes and ignited, causing the rock to fracture. However, such techniques are particularly dangerous for operators, because they involve the use of explosive materials. In addition, such techniques remain dependent on mechanical drills to form holes into which the explosives may be placed.
Still another approach has used projectors that create plasma-hydraulic (or electro-hydraulic), acoustic, and pressure waves to break rock. In such a system, and with reference to FIG. 1, a high voltage is introduced across electrodes 104 immersed in water or some other liquid 108. When the voltage potential between the electrodes 104 is high enough, and the electric field produced by the electrodes 104 exceeds the breakdown electric field of the liquid 108, a conducting plasma channel 112 forms between the two electrodes 104. In addition, a zone of steam or vapor 116 is formed around the plasma channel 112. This zone of vapor 116 propagates outwardly from the channel 112 at a rate that is a function of the power deposited by the electrical current between the electrodes 104 into the channel 112. Power is conducted from the channel 112 to the vapor 116 by thermal conduction and by thermal radiation. A significant portion of the thermal radiation is trapped in the liquid 108 and produces ablation of the bubble wall 120 surrounding the zone of vapor 116, thus adding additional steam 116.
Using plasma-hydraulic methods, very strong pressure waves 124 can be produced as the bubble wall 120 expands against the surrounding liquid 108. By controlling the resulting shock wave, plasma-hydraulic methods may be used to efficiently fragment and break rock in connection with mining and excavation operations. Additional information related to the use of plasma-hydraulic methods can be found in U.S. Pat. Nos. 4,741,405 to Moeny et al., 5,896,938 to Moeny et al., and 6,215,734 to Moeny et al., the disclosures of which are hereby incorporated by reference herein in their entireties.
Although the use of plasma-hydraulic methods to excavate rock are known, the practical implementation of such methods has remained difficult. In particular, the ignition of a series of plasma-hydraulic projectors to excavate an area of rock is difficult, as the cumulative voltage required to ignite the gaps may become exceedingly high. Furthermore, the connection of high voltage cables to the electrodes is problematic, particularly in a wet, dirty mine environment. Also, the reliable mounting of electrode insulators has been problematic. In addition, it would be desirable to reduce the number of electrical cables required to implement a plasma-hydraulic system. Furthermore, it would be desirable to closely integrate plasma-hydraulic projectors and their associated power supplies to allow for the efficient use of plasma-hydraulic methods of breaking rock in a mine environment.
The present invention is directed to solving these and other problems and disadvantages of the prior art. Generally, according to the present invention, one or more groups of plasma-hydraulic projectors are used to break an area of rock. In a typical configuration, each projector within a group includes a reflector, two electrodes defining a gap therebetween, and a connection box in which a high voltage connection between at least a first electrode and a power supply cable may be made. Furthermore, a group of projectors may be interconnected to a common frame that also houses power supply components to form an excavation module. For example, power supply capacitors may be located within the frame, in close proximity to the projectors. Additional components that may be provided as part of the excavation module include trigger circuit transformers and a trigger circuit switch used in connection with the ignition of the projectors.
In accordance with an embodiment of the present invention, the connection box associated with each of the projectors is a water tight housing defining an interior space. The connection box is adapted to receive at least a first electrode of the projector and power supply cables. Interconnections between the at least a first electrode and the power supply cables are made within the interior of the connection box. The entry points of the electrode and the power cables into the connection box are sealed. The connection box allows the high voltage connections between the power supply cables and the electrode to be made quickly and easily. In addition, the connection box provides a space in which the interconnection between the electrode and the power supply cables can be made that is protected from water or other liquids used in connection with the plasma-hydraulic excavation system, and from dirt and debris in the mine environment. In accordance with a further embodiment of the present invention, high voltage connections to both hot and ground electrodes associated with a projector are made within a single connection box.
In accordance with another embodiment of the present invention, the projectors feature an electrode insulator that is mounted in compression to the projector assembly. By mounting the insulator in compression, the reliability and useful life of the insulator is increased as compared to a system that places the insulator in tension.
In accordance with still another embodiment of the present invention, a group of projectors are electrically interconnected to one another in series. In addition, the secondary winding of a trigger transformer is interconnected across the gap of each of the projectors. The primary windings of the trigger transformers are interconnected in parallel to a trigger voltage source to form a trigger circuit. When firing, or ignition of the group of projectors is desired, a trigger voltage source switch connecting the primary windings of the transformers to the trigger voltage source is closed at about the same time that a current source switch connecting a current source to the series connected projector gaps is closed. The voltage supplied across each projector gap by the trigger circuit creates a voltage potential across each projector gap that exceeds the breakdown voltage of the liquid in the gap. Accordingly, a microscopic current channel, or streamer, is established between the electrode pair. At about the same time that the current channel is established, the voltage potential from the current source is at or near a maximum. The current channel, or streamer, then conducts the current from the current source, resulting in ignition of the projectors and creating a shock wave that breaks the rock adjacent to the face of the projector.
According to yet another embodiment of the present invention, multiple current source circuits comprising multiple capacitor banks for supplying current to corresponding groups of projectors are operated from a single current source switch using a single pair of control cables. According to such an embodiment each capacitor bank may comprise a vector inversion circuit. Multiple vector inversion circuits may be connected in parallel with a single current source switch to allow their simultaneous operation. In accordance with an embodiment of the present invention, the current source switch comprises a thyratron.